1. Field of the Invention
This invention pertains to the general field of optical communications and, in particular, to a compensating device for adjusting the optical path length of a wavefront.
2. Description of the Prior Art
The proper functioning of optical devices is often predicated upon the precise control of the optical path length of a wavefront traversing the device. For example, phase-shift interferometers vary the optical path difference between a test beam and a reference beam by translating the reference mirror in a very precise manner along the optical path of the reference beam. Another example, particularly relevant to the preferred embodiment of the invention, lies in the structure and function of optical interleavers.
In dense wavelength division multiplexing (DWDM) optical communication, various frequencies (wavelengths) of 1/xcex laser light are used as carrier signals (channels) and are coupled into the same optical fiber, which acts as a waveguide. Data signals are superimposed over the carrier signals and are transported in the waveguide. An interferometric interleaver is commonly used in the art to separate the channels into distinct optical signals.
Referring to the drawings, wherein like parts are designated throughout with like numerals and symbols, FIG. 1 illustrates schematically a step-phase interferometer used as an interleaver device. A multi-channel optical input W0 is passed through a beam splitter 10 which splits the beam into a first wavefront W1 transmitted toward a Gires-Tournois resonator (GTR) device Mc (also known in the art as an xe2x80x9cetalonxe2x80x9d) and a second wavefront W2 reflected toward a mirror M2. The GTR includes a front surface 12 and a parallel back surface 14 with very low and very high reflectance, respectively. The GTR Mc and the mirror M2 are positioned at distances L1 and L2, respectively, from the interface 16 of the splitter 10. The GTR causes a phase shift in the wavefront W1 which is returned to and partially reflected by the beamsplitter 10 to produce a beam ETCR, in a first output arm of the step-phase interferometer and is partially transmitted to produce a beam ETCT in a second output arm of the interleaver. Similarly, the wavefront W2 is reflected from the mirror M2, is partially transmitted by the beamsplitter 10 to produce a beam ERMT in the first output arm, and is partially reflected to produce a beam ERMR in the second output arm of the interleaver. The notation used in this disclosure, wherein T, R, M and C refer to transmission, reflection, mirror and cavity (resonator), respectively, and the prime symbol (xe2x80x2) refers to the internal optical path, is conventional in the art.
As those skilled in the art would readily recognize, the ability to set very precisely L1-L2 and Lc, the distance between the front and back surfaces 12,14 of the etalon Mc, is critical to the operation of the interleaver. In a 50 GHz interferometric interleaver, for example, the cavity length Lc must be controlled to a precision of a few nanometers, which in the art can only be achieved by using complex and expensive structural features and manufacturing techniques. Furthermore, because structural properties are greatly affected by temperature changes, thermal effects constitute an additional challenge in providing reliable and stable performance of etalons and interleavers.
In view of the foregoing, there is still a need for an extremely accurate and relatively inexpensive way of controlling the optical path length of a wavefront. There is also a need for a path-length adjustment mechanism that is relatively insensitive to thermal effects. The present invention provides simple solutions to that end.
This invention is based on the realization that a parallel glass plate can be used advantageously to fine tune the optical path length of a light wavefront with a degree of accuracy in the order of nanometers. It is known that a parallel glass plate will translate laterally an incident beam along an axis parallel to the incident optical axis by an amount that depends on the distance between the parallel surfaces in the plate, the index of refraction of the glass, and the angle of incidence of the beam. This property is often used to correct the optical axis position in optical apparatus. As illustrated in FIG. 2, the property can also be used to shift laterally the position of the output beam simply by rotating the glass plate and, therefore, changing the angle of incidence of the input beam.
One aspect of the invention consists of combining a parallel glass plate and a rotating fixture and placing the resulting assembly in the optical path of the wavefront requiring fining tuning. As the wavefront passes through the plate, its path length within the glass is affected by the degree to which it is refracted, which in turn is a function of the angle of incidence of the incoming beam. Thus, the optical path length of the wavefront can be changed advantageously by varying its angle of incidence to the plate. Inasmuch as the motion of a rotating fixture can be controlled easily and very precisely with a mechanical actuator, the rotating glass plate of the invention provides a simple and effective way to fine tune the optical path length of any optical device where such length is critical to performance.
According to the preferred embodiment of the invention, a refractive parallel glass plate mounted on a rotating fixture, hereafter referred to as a xe2x80x9cfrequency window,xe2x80x9d is used to precisely control the length of the optical beams in the two arms of an interleaver device. A frequency window is placed in the path of the wavefront W2 reflected by mirror M2 in one arm of the interleaver and the other window is placed in the other arm in the cavity of the etalon Mc (FIG. 1). By rotating each frequency window, the cavity length Lc and the optical path difference (L2xe2x88x92L1) in the two arms of the interleaver can be adjusted to a degree not attainable by mechanical means.
According to another important aspect of the invention, the material used in the construction of the frequency window can be selected so as to produce a thermally stable device. Based on the general relationship between index of refraction, coefficient of thermal expansion and temperature in a medium, it can be shown that the rate of change of the refractive index is a function of temperature. Therefore, by judiciously choosing a material with the appropriate refractive index n, dn/dT, and coefficient of thermal expansion, it is possible to construct a frequency window with negligible temperature dependence.
Those skilled in the art will understand that the same concepts can be used to finely tune the optical path length of light in other applications. For example, the invention may be used to adjust the path length of a wavefront in any interferometric profiler. Therefore, a broad benefit of this invention is the precise control of the length of a light path.
Another important benefit of the invention resides in the fact that it enables the implementation of a thermally stable configuration.
Still another benefit of the invention is the reduction of the complexity and cost of manufacturing highly precise etalons and other devices that require accurate control of the optical path length.
Various other advantages will become clear from the description of the invention in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments, and particularly pointed out in the claims. However, such drawings and descriptions disclose only some of the various ways in which the invention may be practiced.